1. Field of the Invention
This invention pertains generally to gas separation apparatus, and more specifically, to the combination of wet centrifugal separation with electrostatic precipitation for the removal of fine particulate and gas contaminants from an air stream in a compact and essentially continuous process.
2. Description of the Related Art
Industries as diverse as mills, pharmaceutical, chemical, and food processing factories, and cement kilns must all separate contaminants or particulates from an air or gaseous stream. The gases may be a product of combustion, such as present in an exhaust stack, but may also represent other gas streams and may contain such diverse materials as liquid particulates, smoke or dust from various sources, and the like. Separators that must process relatively large volumes of gas are common in power generating facilities and factories.
The techniques used for purification of gas streams have been diverse, including such techniques as filtration, washing, flocculation, centrifugation, and electrostatic precipitation. Each technique has heretofore been associated with certain advantages and disadvantages. These features and limitations have dictated application.
In filtration, particulates are separated through a mechanical filter which selectively traps particles of a minimum size and larger. Unfortunately, flow through a filter is limited by the surface area and cleanliness of the filter, and the size of the openings in the filter. The filter material must be both durable and simultaneously open and porous. In higher volume systems, in corrosive or extreme environments, and in environments with large quantities of fine particulate, filters tend to clog quickly and unpredictably, and present undesirable resistance to the passage of the gas stream. During the period of filter changing or cleaning, which can be particularly tedious, the machine, equipment, or process must be stopped or diverted. This shut-down requires either a duplicate filtration pathway, which may add substantial cost and space requirements, or a shut-down of the machine or process. The limitations present design challenges that have primarily limited this technology to low volume purification.
Washing offers an advantage over dry filtration in presenting the opportunity for selective gas or liquid particulate separation and neutralization, and in reduced gas flow resistance. Unfortunately, the liquid must also be processed, and where there are high levels of particulates, the particulates must be separated from the liquid by yet another process, or the liquid and particulates must be transported to some further industrial or commercial process or disposal location.
Similar to washing, flocculation necessitates the introduction of additional materials that add bulk to the waste stream and unnecessarily complicate the handling and disposal of the contaminants. Furthermore, the flocculating materials must also be provided as raw materials, which may add substantial expense in the operation of such a device, Consequently, flocculation is normally reserved for systems and operations where other techniques have been unsuccessful, or where a particular material is to be removed from the gas stream which is susceptible to specific flocculent that may provide other benefit.
Centrifugation presents opportunity for larger particle removal, such as separation or sand or grit from an air stream. However, centrifugation becomes slower and more complex as the size of the entrained particles or liquids become smaller. Consequently, in applications such as the removal of fly ash from a combustion stream, centrifugation tends to be selective only to relatively large particles, thereby leaving an undesirably large quantity of fine fly-ash in the effluent stream. Furthermore, with larger deviations in particle size, design for adequate separation is more difficult.
Electrostatic precipitators have demonstrated exceptional benefit for contaminants including fly ash, while avoiding the limitations of other processes. For example, unlike centrifugation and filtration, electrostatic precipitators tend to be highly effective at removing particulates of very minute size from a gas stream. The process provides little if any flow restriction, and yet substantial quantifies of contaminants may be removed from the air stream.
When contaminants pass through an electrostatic precipitator, they first pass near precipitator electrodes, which transfer an electrostatic charge to the contaminants. Once charged, the contaminants will be directed by the charge force towards oppositely charged collecting electrodes. The collecting electrodes are frequently in the form of plates having large surface area and relatively small gap between collector plates. The dimensions of the plates and the inter-electrode spacing is a function of the composition of the gas stream electrode potential particulate size of contaminants, anticipated gas breakdown potential, and similar known factors. The selection of dimension and voltage will be made with the goal of gas stream purification in mind, and in gas streams where very fine particulate matter is to be removed, such as with fly ask relatively high voltage potentials and larger plates may be provided. The proper transfer of charge to the particulates and the subsequent electrostatic attraction to collector plates is vital for proper operation. As may be recognized, contaminant cases may not be separated using electrostatic precipitation.
In electrostatic precipitators the collector plates accumulate particulate contaminants. This is by design. As electrically non-conductive particles are deposited, the layers of accumulating particles develop a charge potential gradient through the thickness of the deposited layer, whereby the voltage at the exposed surface decreases in electrical potential, and possibly even reverses charge. When a sufficiently thick layer of electrically non-conductive particles have accumulated to reduce the surface potential, further significant particulate capture becomes difficult or impossible. Disadvantageously then, the conventional plate-type electrostatic separators have certain drawbacks, which include collection efficiency reduction due to high or low resistivity dust accumulation, re-entrainment due to mixing of gas and broken dust layer, leakage of untreated dust from sides of the electrodes, and sweepage due to leakage from below the electrodes over collection hoppers. When the dust resistivity is great enough, the potential gradient through the dust layer formed on the collecting electrodes may locally exceed the layer's breakdown potential. This causes a phenomenon known as “back-corona”, “back-discharge”, “back-ionization”, or “reverse-ionization”, which results in re-entrainment of collected particles in the clean stream. On the other hand, when the resistivity of the dust is low, there is little force to hold it on the collecting electrodes. Not only is the dust held insecurely, but it packs together loosely so that its cohesiveness is also low. Therefore, the dust can be removed from the electrodes by small vibrations or even variations in gas velocities.
Rapping, which is mechanical agitation designed to remove dust from electrodes, leads to a certain amount of re-entrainment into the gas stream. Rapping re-entrainment in severe cases can account for more than 90% of the outlet dust burden. When rapped, poorly cohesive dust tends to break into a cloud of small clumps instead of falling neatly into the hopper as a coherent sheet. As a consequence, much of the dust returns to the gas flow and, unless it is intercepted, will escape from the precipitator outlet, thereby lowering collection efficiency. Consequently, and in spite of the many benefits, electrostatic precipitators have heretofore required a large number of sections that are electrically and mechanically independently operated in a series arrangement to reduce rapping and reintrainment losses to acceptable levels. The present inventor has previously proposed the combination of centrifugation and electrostatic precipitation, in what has heretofore been referred to as an Electro-Core. Several patents illustrate the Electro-Core that are assigned to the present assignee, including U.S. Pat. Nos. 5,591,253; 5,683,494; 5,961,693 and 6,096,118; each which are incorporated herein by reference for their teachings of combined centrifugation and electrostatic precipitation. In these patents, an inlet stream is both centrifuged and electrostatically separated, and a continuous effluent stream provides for the continuous removal of a concentrated stream of contaminant, to reduce or eliminate the need to shut down the process for particulate removal. The resulting separator has the added benefits of reduced size and cost, but provides no particulate collection.
Unfortunately, using the combination of centrifugation and electrostatic separation, there still remains a need for improved removal of particulate, and additional desire to remove contaminant gases, which are presently unremovable using either centrifugation or electrostatic precipitation. What is desired then is a method or apparatus to overcome these limitations of the present Electro-Core precipitators.